The injection molding is extensively used, wherein a molded article is produced by injecting a resin material from an injection machine into a mold while the mold is closed by a mold clamping device. In general, the resin material is supplied to the injection machine in the form of (solid) resin beads. The resin beads are charged into a heating cylinder in which they are plasticized, mixed and kneaded into a material of high fluidity. The flowable material is injected into the mold and, after solidification within the mold, the injected material forms a molded article.
In the case of resin materials as typified by silicone rubber, they are fed into the injection machine in the form of a liquid material and injected from the injection machine into the mold. These materials will be hereinafter referred to as “liquid resin materials” as they are in a liquid state before being supplied into the injection machine.
One example of such liquid resin materials is a two-part liquid resin material. The two-part liquid resin material is composed of a first liquid forming a base resin, and a second liquid forming an additive represented by a hardener. When the base resin and the hardener are mixed together, a mixture will cure or solidify, by the action of the hardener, with a predetermined hardness for several hours to several tens of hours at room temperature or for several tens of seconds to several tens of minuets at a molding temperature. In the case where the hardener is used, it is desirable that the hardener is added with the base resin as late as possible in order to prevent clogging of piping.
For the liquid resin materials that are easy to solidify, various feeding techniques have been proposed. One example of such prior proposals is disclosed, for example, in Japanese Patent (JP-B) No. 4373932.
FIG. 7 hereof shows a general configuration of a liquid resin material feeding device disclosed in JP 4373932B. As shown in this figure, a base resin 201 is pressure fed by a pump 202 into a metering cylinder 203, and a metered amount of base resin is supplied from the metering cylinder 203 to a heating cylinder 205 of an injecting machine 204. On the other hand, a hardener 206 is pressure-fed by a pump 207 into a metering cylinder 208, and a metered amount of hardener is supplied from the metering cylinder 208 into the heating cylinder 205 of the injection machine.
The base resin 201 and the hardener 206 are mixed and kneaded by a screw 209 in the heating cylinder 205 and subsequently injected as indicated by a profiled arrow 211.
Though not described in JP 4373932B, a rotary pump or a reciprocating pump is normally employed for the pumps 202, 207. The reciprocating pump is inexpensive as compared to the rotary pump. The reciprocating pump is available in various forms such as a swash plate pump having plungers driven by a swash plate, and a piston pump having a reciprocating piston in a cylinder. The piston pump is advantageous for its lower cost as compared to the swash plate pump.
The piston pump is, however, disadvantageous for its delivery pressure fluctuation which is incomparably greater than that of the swash plate pump. When the pressure for feeding each material 201, 206 to a corresponding one of the metering cylinders 203, 208 fluctuates, the metered material value also fluctuates. It has, therefore, been recommended for the liquid resin material feeding device to use a rather expensive pump other than the piston pump. However, reduction of the equipment cost is demanded, and, under such conditions, the adoption of the piston pump is recommended.